C-Fos expression in the rat brain after pharmacological stimulation of the rat "mediodorsal" thalamus by means of microdialysis.

In order to visualize target cells of thalamic projections in the rat brain we examined the induction of c-fos messenger RNA and Fos-like immunoreactivity following stimulation of the "mediodorsal" thalamus (midline, mediodorsal and intralaminar nuclei) in freely moving rats. The thalamic neurons were activated through disinhibition by perfusion of the GABAA antagonist bicuculline-methyl chloride via a microdialysis cannula placed in the mediodorsal nucleus of the thalamus. The rats were allowed a recovery period of at least 20 h after surgery before being coupled to the perfusion pump. Cannula implantation with or without 4 h of Ringer perfusion caused hardly any detectable c-fos expression in the brain, but 20 min of bicuculline (0.1 mM) perfusion induced high levels of c-fos messenger RNA and Fos protein expression in the area adjacent to the dialysis membrane, indicating activated thalamic neurons. In situ hybridization as well as immunohistochemical analysis of the frontal cortical areas and limbic structures showed a rapid, specific and transient c-fos expression in the medial and lateral prefrontal cortex, nucleus accumbens, mediodorsal striatum, claustrum, nucleus reticularis of the thalamus and amygdala. The overall spatial distribution of the c-fos response was comparable to the innervation patterns of thalamic efferents known from anatomical tracing experiments. The rats were perfused with Ringer while asleep, but they woke up during treatment with bicuculline and displayed an increase in general behavioural activity, which could be correlated to the amount of bicuculline measurable in the dialysate. Pathological behaviours, such as epilepsy, were not noticeable during bicuculline treatment. These results show that it is possible to selectively activate defined anatomical pathways by pharmacological application of drugs using microdialysis in unanesthetized unrestrained animals and to visualize the transsynaptically activated target neurons of these projections. We conclude that this novel experimental approach is indeed suitable for studying functional anatomical pathways.